As the Doctor said, the "Are You A Bot" field itself affects users only when registering: it prevents automated scripts from creating accounts and infesting the site with spam.
After the Chieftain implemented the system in February, I was easily able to change my own setting using my User CP (in IE). The recent upgrade to vBulletin Version 3.6.7 may have caused some bugs.
For the time being, users that cannot change their own settings should send me a PM: I will then force-feed "No" into their profile.
I'll also move this thread into the Bug Report forum and make Ralle aware of its existence.
~ Thread moved.
[edit]
Ralle made a quick modification: all users' profiles have been edited to change their status to non-bot.